The invention relates to a process for purifying an aqueous caprolactam mixture by extracting the caprolactam with the aid of an organic caprolactam solvent that is not miscible with water in a benzene extraction, and releasing the caprolactam from the organic solvent by means of extraction with water, with the formation of an aqueous caprolactam solution, in a back extraction.
Such a process is known from NL-A-77110150, which describes the so-called benzene extraction of caprolactam from water to benzene and the back extraction from benzene to water. The drawback of this process, however, is that if the columns are more heavily loaded, for example if the caprolactam production is increased, this will adversely affect the extraction yield of the extraction columns employed.
The aim of the invention is now to provide a process in which the extraction steps can be more heavily loaded without loss of extraction yield.
This aim is achieved because at least a separate pre-extraction takes place before the benzene extraction and/or the back extraction, in which, in a pre-extraction before the benzene extraction, an organic solvent that is not miscible with water is used for the extraction and, in a pre-extraction before back extraction, water is used for the extraction.
This makes it possible to increase the load of any extraction step by 20% with only one extra theoretical tray; it moreover even leads to an improved extraction yield. Purifying caprolactam by means of benzene extraction and back extraction often implies a bottleneck in the production of caprolactam, because the extraction columns are unable to process the increasing caprolactam productions. This problem can now be simply solved even without having to install extra equipment, for example columns, by causing a pre-extraction to take place. New extraction columns are moreover expensive and it is often difficult to incorporate them in an existing installation.
This pre-extraction can take place in for example a mixer/settler. A mixer/settler is an apparatus comprising a mixing part and a settling part. The liquids are combined in the mixing part and energy is supplied to them, for example by means of a stirrer. This results in the formation of droplets of one of the liquids in the other, a dispersion. The dispersion then remains in the settling part for a sufficient length of time for the droplets to coalesce, preferably with a laminar flow.
Suitable mixer/settlers are the box-type mixer/settlers, the IMI, the xe2x80x9cGeneral Millsxe2x80x9d or the Kemira mixer/settlers, described in xe2x80x9cLiquid-Liquid Extraction Equipmentxe2x80x9d by Godfrey J. C. and Slater M. J., Ed. Wiley, COP (1994), Ch.I, pp. 294-297.
It is also possible to place an extension on a column, which extension may optionally also be widened.
Caprolactam-containing mixtures can for example be extracted with the aid of extraction columns. Columns fitted with rotating internally installed elements, known as xe2x80x98rotating disc columnsxe2x80x99 (RDCs), and elements for pulsing the liquid column are suitable for use as extraction columns. Pulsing columns with packing bodies, pulsing columns with sieve trays, asymmetrical rotating disc contactors (ARD), Scheibel columns and Kxc3xchni columns are also suitable. In general it will suffice to carry out the process in a column comprising 3-25 theoretical trays, depending on the desired degree of purification. It is of course possible to increase the number of trays if so desired. The extraction process is preferably carried out in countercurrent mode, because the extraction will then proceed in the most efficient, and hence an economic, manner.